A patient who has suffered extensive skin loss or injury is immediately threatened by infection and by excessive loss of fluids. To meet both of these needs, a large skin wound must be closed promptly by some type of membrane. The most direct method of accomplishing this purpose is by transplanting partial-thickness sections of skin to the wound, thereby sealing the wound and preventing fluid loss and infection.
The transplanted section of skin can be removed ("harvested") from an animal of another species. This type of transplant is referred to as a xenograft. However, a xenograft suffers from the disadvantage that the transplanted skin is rejected and can only serve to cover the wound for three to five days. Consequently, a xenograft can only serve as a stopgap while the patient's skin slowly heals.
The transplanted section of skin can also be harvested from human cadavers. This type of transplant is referred to as an allograft or homograft. However, cadaveric skin is in short supply, and allografts are often, like xenografts, rejected. Although immunosuppressive drugs can increase the period of time which an allograft may cover a wound, they also leave the patient vulnerable to infection. Allografts also suffer from the disadvantage that they expose the patient to the risk of transmission of diseases such as hepatitis and AIDS.
The most desirable form of transplant is an autograft, in which skin from an undamaged area of the patient or identical twin is harvested and used to cover the wound. The risk of rejection and disease transmission is thereby eliminated, and the transplanted skin proliferates to form a new layer of dermis and epidermis.
The harvesting operation is a painful, invasive process, which causes scarring. It should therefore be kept to a minimum. In addition, a severely burned patient may suffer skin loss or damage on nearly all of his or her body. This may severely limit the amount of healthy, intact skin that is available for autografting. When this occurs, xenografts or homografts may be placed across the entire wound surface to control infection and dehydration; they are gradually replaced as autografts become available. Autografts may be harvested repeatedly from a donor site. In such an operation, an area of xenograft or homograft is removed and discarded, and replaced by an autograft. Each donor site must be allowed to heal before another autograft is removed from it; this requires a substantial delay, and prolongs the recovery of the patient. Furthermore, the quality of the skin graft diminishes with each successive harvest.
Consequently, much effort has been spent to create a skin substitute for the massively burned patient with limited donor sites. Attempts have been made to manufacture artificial skin from both biologic and synthetic materials with variable results. An acceptable skin substitute should provide both the components and functional results of normal skin. Two important components of the skin are the epidermis and dermis. The epidermis is the outer layer of skin. It consists of cells at various stages of differentiation and maturity. Basal cells are located at the lowest level (adjacent to the dermis) and are the least differentiated. The dermis is located below the epidermis and comprises mesenchymal cells and blood vessels. The junction between the dermis and epidermis is referred to as the basement membrane and is responsible for one of the most important functional results of normal skin, namely the tight adhesion of the dermis to the epidermis. This tight adhesion adds strength and durability to the skin and prevents "shearing" of the epidermis. "Shearing" is the "rubbing off" of the epidermis when lateral forces are applied to the skin, and can result in blistering and skin fragility.
One of the most promising skin substitutes is a synthetic bilayer membrane (hereinafter collectively referred to as "CG bilayer"). This membrane comprises a bottom layer (hereinafter referred to as "CG matrix") which is a highly porous lattice made of collagen and glycosaminoglycan. The outer layer is a membrane semipermeable to moisture and impermeable to infectious agents such as bacteria. The CG lattice serves as a supporting or scaffolding structure into which blood vessels and mesenchymal cells migrate from below the wound, a process referred to as "infiltration". Infiltration is responsible for creating a new dermis, referred to as the "neodermis", which replaces the CG matrix as it biodegrades. Epithelial cells from undamaged skin surrounding the edges of the wound migrate into CG matrix to create a new epidermis, referred as the "neoepidermis". Because burns and other skin wounds tend to be shallow, mesenchymal cells need not migrate very far to create a neodermis. However, burns often cover large areas of a patient's body surface. Consequently, epithelial cells often must migrate great distances to adequately close a wound. As a result, thin skin grafts are required to close the wound. Consequently, a need exists for new procedures which can hasten the coverage of the CG matrix with a neoepidermal layer.
A second promising technology for manufacturing and applying artificial skin is referred to as cultured epithelial autograft (hereinafter referred to as "CEA"). In this method split thickness skin samples are harvested from a site on the patient's body surface that is wound free. The epithelial cells from this graft are grown in culture to give epithelial sheets that are applied directly to the wound bed, basal side down.
The CEA method suffers from the limitation that it only applies a neoepidermis to the wound bed. There is no dermis or basement membrane present at the time of application, and, therefore, no basement membrane. Thus, there is nothing to secure the neoepidermis to the underlying tissue, resulting in poor take rates for CEA sheets applied directly onto wound beds. This is evidenced by shearing and blistering of the transplanted CEA. Consequently, efforts have been made to use dermal substrates, such as cadaveric skin to improve take rates. However, allograft rejection, the risk of disease transmission and limited availability of cadaveric skin are serious limitations on the usefulness of this technique.
Despite the promise of CEA as a technique for treating wounds, improvements are needed if this technique is to adequately meet the needs of patients with wounds covering large portions of their bodies. Take rates need to be improved without incurring the limitations and risks involved in using cadaveric skin. Furthermore, a patient's wounds must either be exposed or temporarily covered during the approximately three week period during which the CEA sheets are being grown.